Device-to-device (D2D) discovery is a process for determining whether a D2D-enabled user equipment (UE) is in proximity to another D2D-enabled UE. The discovering D2D-enabled UE determines, based on the D2D discovery process, whether the other D2D-enabled UE is interested in pairing with the discovering D2D-enabled UE. The other D2D-enabled UE is interested in the discovering D2D-enabled UE in case the proximity to the other D2D-enabled UE needs to be known by one or more certified applications in the discovering D2D-enabled UE.
As an example, a social networking application may be enabled to use a D2D discovery feature. The D2D discovery process enables the D2D-enabled UE of a given user of the social networking application to discover D2D-enabled UEs of friends of the given user and enables the D2D-enabled UE of the given user to be discovered by the D2D-enabled UEs of the friends of the given user. As another example, the D2D discovery process enables the D2D-enabled UE of a given user of a discovery application to discover nearby shops or restaurants of interest.
D2D communications in a frequency division duplex (FDD) system are performed on an uplink (UL) frequency. A dedicated signaling scheme between a D2D UE and a base station (BS) (or an evolved Node B (eNB)) is one method for allocating a resource(s) to the D2D UE for transmission. A D2D UE interested in transmission sends a request for a dedicated resource(s) to an eNB, and the eNB allocates a dedicated resource(s) for transmission to the D2D UE. The request and resource allocation are performed in a connected state where the UE and the eNB are connected with each other. The allocated resource(s) are semi-permanent in nature, and the UE maintains the connected state until the allocated resource(s) are consumed. A D2D UE transmitting D2D packets (discovery or data packets) may need to receive D2D packets (discovery or data packets) from other D2D UEs. In a connected state, a UE needs to continue monitoring downlink (DL) frequencies in order to receive, at least, a packet data common control channel (PDCCH) that is transmitted from an eNB. Accordingly, a D2D UE, upon intending to receive D2D packets on a UL frequency in a connected state, performs the following operations:                1. The D2D UE should not only transmit D2D packets on the UL frequency but receive D2D packets as well.                    a. This does not matter when the D2D UE is able to receive D2D packets through D2D resources unused for D2D transmission.                        2. However, the D2D UE should transmit control data or data packets to the eNB on the UL frequency and receive D2D packets on the UL frequency.                    a. This is not a critical issue when D2D resources are separate from resources for communications with the eNB.            b. In case D2D resources and resources for communications with the eNB are frequency division multiplexed in the same subframe, however, the D2D resources are monitored when the UE is not scheduled to communicate with the eNB.            c. The UE should receive a PDCCH on a DL frequency and receive D2D packets on a UL frequency. No UE provides a capability that allows for simultaneous reception on both a DL frequency and a UL frequency.                        
Accordingly, this is important because the UE needs to monitor DL frequencies regarding PDCCHs even when there is no data scheduled on the UL or DL frequency.
The UE may be in a connected state for communications with the eNB or in a connection for allocating dedicated resources for D2D. The UE, once entering into a connected state, may remain in the connected state for a very long time due to a long period in discovery transmission and the feature of discovery transmission being periodically performed. In such case, the UE may fail to monitor D2D transmissions. Thus, a need exists for an apparatus and method for monitoring D2D transmission in a connected state.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.